1. Field of the Invention
This invention is directed to RF cellular and personal communications systems. More particularly, it is directed to base stations for such systems operating in a multipath transmission and reception environment.
2. Description of the Related Art
RF cellular and personal communication service systems (hereinafter referred to collectively as "PCS systems") typically operate with one or more fixed base stations and a number of low-powered portable terminals. When a portable terminal is used, it establishes a communication session with a base station. Although the portable terminal communicates only with the base station, the base station may service a number of portable terminals at the same time. The base station may provide a virtual connection between two currently active portable terminals, or it may provide a connection between the portable terminal and a conventional telephone network or the like.
Radio waves typically travel from transmitter to receiver in several different modes. For example, ground wave or surface wave propagation occurs when radio waves impact the ground and propagate toward the receiver through the Earth's surface due to its electrical conductivity. Sky wave propagation occurs when space-bound radio waves from a transmitter are refracted in the Earth's ionosphere and directed back toward the receiver. Finally, space wave propagation includes the direct, line-of-sight transmission of radio waves from transmitter to receiver as well as reflected waves which bounce off the Earth's surface, natural features or man-made objects one or more times before they reach the receiver. Due to the particular dielectric characteristics of the Earth's surface and ionosphere when they serve as transmission media, the primary mode of transmission in PCS systems is usually space wave propagation.
Since radio signals travelling in space propagation mode may be reflected one or more times before reaching the receiver. For example, FIG. 1 shows a typical personal telephone system in which two portable terminals 20 and 22 receive signals from a base station 24 by both line-of-sight propagation and by scattering off a building 26 and a car 28, respectively. Since the distance travelled by a line-of-sight signal and a scattered signal (or by two scattered signals travelling on different paths) will differ, the signals will be out of phase with one another at the receiver. If the phase difference is an even multiple of .pi. (corresponding to a delay of 360.degree., 720.degree., etc.), the result is constructive interference in which the composite received signal is reinforced. Small multiples corresponding to a short delay simply increase the effective signal strength of the composite received signal; however, if the transmitted signal is modulated with, e.g., digital data, large multiples corresponding to long delays can result in an "echo" effect which blurs the boundaries between adjacent modulated fields. This effect is known as inter-symbol interference.
On the other hand, if the phase difference is an odd multiple of .pi. (corresponding to a 180.degree., 540.degree., etc. phase difference), the two signals will be opposite in phase to one another, and the received signals will completely cancel out one another. Of course, phase differences that are not integer multiples of .pi. will result in partial cancellation of the received signal, a lower level of inter-symbol interference, a reduction in the signal-to-noise ratio of the received signal, and the like.
One method of compensating for the destructive effects of multipath wave propagation as described above involves the use of diversity, or choosing one of two or more different transmission paths which is superior in quality. The most common diversity methods are frequency diversity, in which the best of a number of signals transmitted at different frequencies is used; polarization diversity, in which the better of a horizontally polarized signal and a vertically polarized signal is used; and space diversity or antenna diversity, in which the best signal transmitted (or received) using multiple antennas separated by a number of wavelengths is used.
Antenna diversity has proven to be the most viable means of multipath propagation compensation in PCS systems; however, its implementation is constrained by a number of factors. For example, portable terminals in PCS systems are typically hand-held or otherwise small-scale units which do not readily permit effective physical separation of multiple antennas. Moreover, the use of antenna diversity at the receiver requires the use of accompanying circuitry to select the optimal received signal, thus further increasing the size as well as the power consumption and cost of the portable terminal.
For these reasons, the use of antenna diversity at the base station side is preferable. Although this technique provides the lowest cost, size and power consumption characteristics for the portable terminals, it requires relatively sophisticated signal path control at the base station for reasons that will soon become apparent.
The capacity of most PCS communication standards is too limited for the system to be used in a dedicated, one user per communication channel mode. For example, the Digital European Cordless Telephone (DECT) standard provides twelve speech channels per RF channel; the GSM standard provides eight; and the Personal Handy Phone (PHS), Personal Digital Cellular (PDC) and IS54 digital standards permit only three each. To be practical, therefore, some technique must be used to increase the information-carrying capacity of signals conforming to such standards.
Two such methods are time division multiplexing (TDM) and frequency division multiplexing (FDM). A TDM signal is divided into sequential frames, and multiple information signals (e.g., user voice signals) are each assigned a different, fixed time slot within a frame. Each of the information signals is repetitively sampled at a point corresponding to its time slot, and the frame based on the samples is transmitted. When received, portions of the TDM signal corresponding to time slots of each information signal are recombined with each other to reconstruct the original signals.
On the other hand, an FDM signal contains a number of information signals which originally occupied the same frequency band. The frequencies of the different information signals are offset so they do not overlap with one another and distributed within the range of the FDM signal bandwidth. A familiar example of an FDM signal is the AM radio band, in which an FDM signal having a bandwidth of 1070 kHz (from 535 to 1605 kHz) can simultaneously carry 107 signals each having a 5 kHz bandwidth, where each signal is separated from adjacent signals by 10 kHz.
These two techniques can of course be combined for maximum information carrying capacity. For example, a PCS system which uses an FDM signal frequency multiplexing four signals, each of which is a TDM signal time multiplexing four user information signals can effectively support sixteen users as shown in TABLE 1:
TABLE 1 ______________________________________ Frequency Channel Time Slot A B C D ______________________________________ I User 1 User 5 User 9 User 13 II User 2 User 6 User 10 User 14 III User 3 User 7 User 11 User 15 IV User 4 User 8 User 12 User 16 ______________________________________
If such a system is implemented using antenna diversity, frequent reallocation of users and time slots among frequency channels may be necessary. For example, consider an antenna diversity system using two antennas 30 and 32 as shown in FIG. 1. Users 5 and 7 may be at physical locations where antenna 30 provides the best communication path, and User 6 may be at a position where antenna 32 provides superior results. In this case, Frequency Channel B must be directed to antenna 30 during time slot I, redirected to antenna 32 during time slot II, and then directed back to antenna 30 during time slot III.
This switching must be done with similar allocations occurring simultaneously on the other frequency channels. Therefore, in any given time slot, a particular antenna may be transmitting all frequency channels, only some of the channels, or no channels at all. Generally speaking, the base station may be required to move from any arbitrary allocation of frequency channels to antennas at one time slot to a completely different and unpredictable allocation at the next time slot.
Prior art frequency division multiplexers such as the one shown in FIG. 2 are unable to perform such a function. In such multiplexers, N TDM digital quadrature input channels C.sub.1 -C.sub.N are each fed to a respective quadrature modulator 34.sub.1 -34.sub.N. In the modulators, the respective input channel C.sub.1 -C.sub.N is used to modulate a carrier signal F.sub.1 -F.sub.N having a unique frequency from an associated local oscillator 36.sub.1 -36.sub.N. The outputs of the modulators 34.sub.1 -34.sub.N are fed to a summing amplifier 38 which combines the signals to produce an FDM output signal T, where each of the multiplexed components of the FDM signal T is a digital TDM signal.
One solution to the above problem is to arbitrarily assign each of the frequency channels to one of the antennas. For example, assume in the arrangement of TABLE 1 that Frequency Channels A and B are assigned to antenna 30, while Frequency Channels C and D are assigned to antenna 32. This avoids the need to dynamically reassign frequency channels to antennas; however, it reduces system efficiency in the process. For example, during Time Slot II, if antenna 30 provides the best communication path for Users 2 and 6 and antenna 32 provides the best communication path for Users 10 and 14, all is well. If, however, User 10 is also best served by antenna 30, the system cannot accommodate all three users with the two frequency channels assigned to antenna 30, and one of the users' signals would be transmitted under less than optimal conditions.